Integration with Existing Apps

React Native is great when you are starting a new mobile app from scratch. However, it also works well for adding a single view or user flow to existing native applications. With a few steps, you can add new React Native based features, screens, views, etc.

The specific steps are different depending on what platform you're targeting.

iOS (Objective-C)

iOS (Swift)

Android (Java)

Key Concepts

The keys to integrating React Native components into your iOS application are to:

Set up React Native dependencies and directory structure.

Understand what React Native components you will use in your app.

Add these components as dependencies using CocoaPods.

Develop your React Native components in JavaScript.

Add a RCTRootView to your iOS app. This view will serve as the container for your React Native component.

Start the React Native server and run your native application.

Verify that the React Native aspect of your application works as expected.

The keys to integrating React Native components into your Android application are to:

Set up React Native dependencies and directory structure.

Develop your React Native components in JavaScript.

Add a ReactRootView to your Android app. This view will serve as the container for your React Native component.

Start the React Native server and run your native application.

Verify that the React Native aspect of your application works as expected.

Prerequisites

Follow the instructions for building apps with native code from the Getting Started guide to configure your development environment for building React Native apps for iOS.

1. Set up directory structure

To ensure a smooth experience, create a new folder for your integrated React Native project, then copy your existing iOS project to a /ios subfolder.

Follow the instructions for building apps with native code from the Getting Started guide to configure your development environment for building React Native apps for Android.

1. Set up directory structure

To ensure a smooth experience, create a new folder for your integrated React Native project, then copy your existing Android project to an /android subfolder.

2. Install JavaScript dependencies

Go to the root directory for your project and create a new package.json file with the following contents:

It is technically possible not to use CocoaPods, but that would require manual library and linker additions that would overly complicate this process.

Adding React Native to your app

Assume the app for integration is a 2048 game. Here is what the main menu of the native application looks like without React Native.

Assume the app for integration is a 2048 game. Here is what the main menu of the native application looks like without React Native.

Command Line Tools for Xcode

Install the Command Line Tools. Choose "Preferences..." in the Xcode menu. Go to the Locations panel and install the tools by selecting the most recent version in the Command Line Tools dropdown.

Configuring CocoaPods dependencies

Before you integrate React Native into your application, you will want to decide what parts of the React Native framework you would like to integrate. We will use CocoaPods to specify which of these "subspecs" your app will depend on.

The list of supported subspecs is available in /node_modules/react-native/React.podspec. They are generally named by functionality. For example, you will generally always want the Coresubspec. That will get you the AppRegistry, StyleSheet, View and other core React Native libraries. If you want to add the React Native Text library (e.g., for <Text> elements), then you will need the RCTTextsubspec. If you want the Image library (e.g., for <Image> elements), then you will need the RCTImagesubspec.

You can specify which subspecs your app will depend on in a Podfile file. The easiest way to create a Podfile is by running the CocoaPods init command in the /ios subfolder of your project:

$ pod init

The Podfile will contain a boilerplate setup that you will tweak for your integration purposes. In the end, Podfile should look something similar to this:

# The target name is most likely the name of your project.
target 'NumberTileGame'do# Your 'node_modules' directory is probably in the root of your project,# but if not, adjust the `:path` accordingly
pod 'React', :path => '../node_modules/react-native', :subspecs => [
'Core',
'CxxBridge', # Include this for RN >= 0.47'DevSupport', # Include this to enable In-App Devmenu if RN >= 0.43'RCTText',
'RCTNetwork',
'RCTWebSocket', # Needed for debugging'RCTAnimation', # Needed for FlatList and animations running on native UI thread# Add any other subspecs you want to use in your project
]
# Explicitly include Yoga if you are using RN >= 0.42.0
pod 'yoga', :path => '../node_modules/react-native/ReactCommon/yoga'# Third party deps podspec link
pod 'DoubleConversion', :podspec => '../node_modules/react-native/third-party-podspecs/DoubleConversion.podspec'
pod 'glog', :podspec => '../node_modules/react-native/third-party-podspecs/glog.podspec'
pod 'Folly', :podspec => '../node_modules/react-native/third-party-podspecs/Folly.podspec'end

If this fails with errors mentioning xcrun, make sure that in Xcode in Preferences > Locations the Command Line Tools are assigned.

If you get a warning such as "The swift-2048 [Debug] target overrides the FRAMEWORK_SEARCH_PATHS build setting defined in Pods/Target Support Files/Pods-swift-2048/Pods-swift-2048.debug.xcconfig. This can lead to problems with the CocoaPods installation", then make sure the Framework Search Paths in Build Settings for both Debug and Release only contain $(inherited).

Code integration

Now we will actually modify the native iOS application to integrate React Native. For our 2048 sample app, we will add a "High Score" screen in React Native.

The React Native component

The first bit of code we will write is the actual React Native code for the new "High Score" screen that will be integrated into our application.

1. Create a index.js file

index.js is the starting point for React Native applications, and it is always required. It can be a small file that requires other file that are part of your React Native component or application, or it can contain all the code that is needed for it. In our case, we will just put everything in index.js.

2. Add your React Native code

In your index.js, create your component. In our sample here, we will add simple <Text> component within a styled <View>

RNHighScores is the name of your module that will be used when you add a view to React Native from within your iOS application.

The Magic: RCTRootView

Now that your React Native component is created via index.js, you need to add that component to a new or existing ViewController. The easiest path to take is to optionally create an event path to your component and then add that component to an existing ViewController.

We will tie our React Native component with a new native view in the ViewController that will actually host it called RCTRootView .

1. Create an Event Path

You can add a new link on the main game menu to go to the "High Score" React Native page.

2. Event Handler

We will now add an event handler from the menu link. A method will be added to the main ViewController of your application. This is where RCTRootView comes into play.

When you build a React Native application, you use the React Native packager to create an index.bundle that will be served by the React Native server. Inside index.bundle will be our RNHighScore module. So, we need to point our RCTRootView to the location of the index.bundle resource (via NSURL) and tie it to the module.

We will, for debugging purposes, log that the event handler was invoked. Then, we will create a string with the location of our React Native code that exists inside the index.bundle. Finally, we will create the main RCTRootView. Notice how we provide RNHighScores as the moduleName that we created above when writing the code for our React Native component.

First import the RCTRootView header.

#import <React/RCTRootView.h>

The initialProperties are here for illustration purposes so we have some data for our high score screen. In our React Native component, we will use this.props to get access to that data.

Note that RCTRootView initWithURL starts up a new JSC VM. To save resources and simplify the communication between RN views in different parts of your native app, you can have multiple views powered by React Native that are associated with a single JS runtime. To do that, instead of using [RCTRootView alloc] initWithURL, use RCTBridge initWithBundleURL to create a bridge and then use RCTRootView initWithBridge.

First import the React library.

import React

The initialProperties are here for illustration purposes so we have some data for our high score screen. In our React Native component, we will use this.props to get access to that data.

Note that RCTRootView bundleURL starts up a new JSC VM. To save resources and simplify the communication between RN views in different parts of your native app, you can have multiple views powered by React Native that are associated with a single JS runtime. To do that, instead of using RCTRootView bundleURL, use RCTBridge initWithBundleURL to create a bridge and then use RCTRootView initWithBridge.

When moving your app to production, the NSURL can point to a pre-bundled file on disk via something like [[NSBundle mainBundle] URLForResource:@"main" withExtension:@"jsbundle"];. You can use the react-native-xcode.sh script in node_modules/react-native/scripts/ to generate that pre-bundled file.

When moving your app to production, the NSURL can point to a pre-bundled file on disk via something like let mainBundle = NSBundle(URLForResource: "main" withExtension:"jsbundle"). You can use the react-native-xcode.sh script in node_modules/react-native/scripts/ to generate that pre-bundled file.

3. Wire Up

Wire up the new link in the main menu to the newly added event handler method.

One of the easier ways to do this is to open the view in the storyboard and right click on the new link. Select something such as the Touch Up Inside event, drag that to the storyboard and then select the created method from the list provided.

Test your integration

You have now done all the basic steps to integrate React Native with your current application. Now we will start the React Native packager to build the index.bundle package and the server running on localhost to serve it.

App Transport Security is good for your users. Make sure to re-enable it prior to releasing your app for production.

2. Run the packager

To run your app, you need to first start the development server. To do this, simply run the following command in the root directory of your React Native project:

$ npm start

3. Run the app

If you are using Xcode or your favorite editor, build and run your native iOS application as normal. Alternatively, you can run the app from the command line using:

# From the root of your project
$ react-native run-ios

In our sample application, you should see the link to the "High Scores" and then when you click on that you will see the rendering of your React Native component.

Here is the native application home screen:

Here is the React Native high score screen:

If you are getting module resolution issues when running your application please see this GitHub issue for information and possible resolution. This comment seemed to be the latest possible resolution.

See the Code

You can examine the code that added the React Native screen to our sample app on GitHub.

You can examine the code that added the React Native screen to our sample app on GitHub.

1. Create a index.js file

index.js is the starting point for React Native applications, and it is always required. It can be a small file that requires other file that are part of your React Native component or application, or it can contain all the code that is needed for it. In our case, we will just put everything in index.js.

2. Add your React Native code

In your index.js, create your component. In our sample here, we will add simple <Text> component within a styled <View>:

3. Configure permissions for development error overlay

If your app is targeting the Android API level 23 or greater, make sure you have the permission android.permission.SYSTEM_ALERT_WINDOW enabled for the development build. You can check this with Settings.canDrawOverlays(this);. This is required in dev builds because React Native development errors must be displayed above all the other windows. Due to the new permissions system introduced in the API level 23 (Android M), the user needs to approve it. This can be achieved by adding the following code to your Activity's in onCreate() method.

Finally, the onActivityResult() method (as shown in the code below) has to be overridden to handle the permission Accepted or Denied cases for consistent UX. Also, for integrating Native Modules which use startActivityForResult, we need to pass the result to the onActivityResult method of our ReactInstanceManager instance.

The Magic: ReactRootView

Let's add some native code in order to start the React Native runtime and tell it to render our JS component. To do this, we're going to create an Activity that creates a ReactRootView, starts a React application inside it and sets it as the main content view.

If you are targetting Android version <5, use the AppCompatActivity class from the com.android.support:appcompat package instead of Activity.

A ReactInstanceManager can be shared by multiple activities and/or fragments. You will want to make your own ReactFragment or ReactActivity and have a singleton holder that holds a ReactInstanceManager. When you need the ReactInstanceManager (e.g., to hook up the ReactInstanceManager to the lifecycle of those Activities or Fragments) use the one provided by the singleton.

Next, we need to pass some activity lifecycle callbacks to the ReactInstanceManager and ReactRootView:

This allows JavaScript to control what happens when the user presses the hardware back button (e.g. to implement navigation). When JavaScript doesn't handle the back button press, your invokeDefaultOnBackPressed method will be called. By default this simply finishes your Activity.

Finally, we need to hook up the dev menu. By default, this is activated by (rage) shaking the device, but this is not very useful in emulators. So we make it show when you press the hardware menu button (use Ctrl + M if you're using Android Studio emulator):

Test your integration

You have now done all the basic steps to integrate React Native with your current application. Now we will start the React Native packager to build the index.bundle package and the server running on localhost to serve it.

1. Run the packager

To run your app, you need to first start the development server. To do this, simply run the following command in the root directory of your React Native project:

$ yarn start

2. Run the app

Now build and run your Android app as normal.

Once you reach your React-powered activity inside the app, it should load the JavaScript code from the development server and display:

Creating a release build in Android Studio

You can use Android Studio to create your release builds too! It’s as easy as creating release builds of your previously-existing native Android app. There’s just one additional step, which you’ll have to do before every release build. You need to execute the following to create a React Native bundle, which will be included with your native Android app: